1. Field of the Invention
The present invention relates to a call collision sidestep system which carries out connections in response to simultaneous call requests between opposed communication equipment units, such as terminal equipment units, in an exchange service of an integrated services digital network (ISDN).
2. Description of the Related Art
A plurality of switched line services are supplied, in a switched line applicable to data communication, and an exchange service in ISDN networks is utilized as one such service.
In general, communication equipment in use for exchange services is connected with other ("opposed") such equipment, which communicate with each other utilizing exchange services, to effect communication at a time when a call request is produced.
the above exchange services are often employed for an emergency backup or the like and, in such a circumstance, the time duration from an occurrence of a connect request to a connect terminate (in a state of data set available) is required to be curtailed.
Nevertheless, in such an above-described ISDN network or in a public switched telephone network (PSTN), when the communication equipment units each internally carry out a connect request to a distant station and two such distant, or opposed, units do so simultaneously, a collision of the incoming call from the one unit with the outgoing call from the other unit occurs, and prevents establishing the requested connection.
For this reason, even if the communication equipment unit internally carries out a connect request to distant station equipment simultaneously, it is required to use a method that can sidestep the collision of incoming and outgoing calls and connect with any opposed unit between the communication equipment units.
In a conventional method, when two communication equipment units connected with an ISDN network carry out respective connect requests to the opposed equipment unit simultaneously, both communication equipment units are put in an answer waiting state with respect to the opposed station, relative to a call setup signal outgoing from an intra-office station, and each is in a state such that a network connection does not terminate (i.e., as would occur in a state that a collision of the incoming and outgoing calls has occurred).
When such a call collision occurs, a connect request is withdrawn using conventional methods and, after a definite time lapse, a connect request is made again.
Among the above-described methods for determining a definite time, a first method is such that the time lapse is determined by each communication equipment unit in a pseudo-random way and a second method is such that different definite time lapses are determined in advance to set up the calls between the respective communication equipment units and each thus is able to connect with the other, or the like.
Concerning the above first method, there is a problem that, whenever an incoming and outgoing call collision occurs, a retry (i.e., call abort signal and re-transmission of the outgoing signal) is repeated, every time, and thus it takes a long time for a connection, a connect time is not specified, and so on.
Concerning the above second method, there is a problem that it is necessary to control the setup time on all the communication equipment units that are connected to the network and thus when the equipment units having a long setup time are connected to each other, it takes a longer time for them to be connected together.
In the prior art, when the communication equipment units are connected with each other and an incoming and outgoing call collision occurs, it takes a long time for one to be connected with another and, as a result, it takes too much time from a communication generation request time to a communication enable time.
Such a state of affairs produces a serious problem in an emergency communication, such as a backup of a based circuit (which is usually used by a leased line), or it enhances deterioration or decline of response characteristics all over the system.